The present invention relates to pressure switches and particularly those of the type intended for operation at a relatively low gauge pressure on the order of 1 psig with a narrow or tightly controlled differential between trip and reset or between actuation and deactuation. Such switches find common application in automatic clothes washing machines where it is desired to control certain machine electrical functions in response to the water level or pressure head sensed in the washing receptacle or drum. Typically in mass produced automatic clothes washing machines a sensing port is provided in the washing tub or drum with a tube connected from the sensing port to the pressure switch. The pressure in the switch sensing cavity is commonly applied in such known designs to an elastomeric diaphragm which has a rigid backing member or actuator attached thereto for applying the sum of the pressure forces acting on the diaphragm to an electrical switching mechanism.
A known pressure switch used for automatic clothes washing machine water level sensing is illustrated in FIG. 7 and has a pressure responsive diaphragm 1 disposed in a housing 2 forming a pressure sensing cavity 3 connected to inlet port 4. The diaphragm has a backing plate 5 for applying the summation of the pressure forces acting over the diaphragm 1 to a switch actuation rod 7 which in turn is connected for moving a switch actuation blade 6. The known pressure switch of FIG. 7 has the preload or bias applied directly to the diaphragm backing plate via rod 7. The actuator 5 is preloaded by the force of rod 7 which is acted upon by a preload spring 8 which is adjustable for varying the preload on the diaphragm for calibration purposes. The blade 6 has a separate tang formed therein with a double sided moveable contact 9 mounted thereon for switching between the moveable contact and stationary side contacts 10; and, an over-center spring 12 provides a snap action to the switch.
The problem encountered with the prior art pressure switches, such as switches of the design shown in FIG. 7, is that, upon calibration of the switch by adjustment of the preload on the diaphragm actuator, the differential between actuation and deactuation of the switch is varied as the preload was varied on the diaphragm by virtue the changed position of the actuator blade 6 and the tolerances on the location and spacing of the side contacts 10 and 11. During assembly and calibration, it has been found difficult to control the differential between actuation and deactuation, or trip-to-reset, over a range of pressure calibrations and tolerances on assembly of the switch contacts into the housing.
Thus, it has long been desired to find a way or means of providing a pressure switch for low pressure gauge settings, such as encountered in automatic clothes washers for water level sensing, and to provide accuracy in calibration and trip to reset and yet accomplish these functions with a minimum manufacturing cost.
The present invention provides an improved pressure switch for relatively low gauge pressure actuation and deactuation with reduced differential or variance between trip and reset and provides for accurate setting of the trip-to-reset during manufacturing.
It is an object of the present invention to provide a relatively low gauge pressure sensing pressure switch which may be calibrated for pressure actuation and independently set for the differential between trip and reset separately without interaction or of one effecting the other.
The present invention provides a pressure switch having a pressure responsive member moveable in response to changes in pressure in the switch sensing cavity formed in the housing and operable to move a cantilevered switch contact blade spring having the middle portion contacting and biasing the pressure responsive member. The blade spring includes an over-center snap mechanism which causes a tang formed in the blade spring to open and close a moveable contact mounted on the tang against a stationary switch contact. An adjustment means on the housing is operable to change the position of the free end of the blade spring to vary the bias on the pressure responsive means for changing the pressure level at which the switch actuates. The positioning of the stationary contact with respect to the moveable contact determines the differential or trip-to-reset pressure for the switch independently of the actuation point setting.